Neutron source



y 1951 ,1. M. TEHAYER ET AL 2,991,367

NEUTRON SOURCE Original Filed July 30, 1949 73 SAMPLE 72 DETECTORINVENTORS ROBERT E. FEARON JEAN M. THAY ER ATTORNEY Patent 0135C?NEUTRON SOURCE Jean M. Thayer and Robert E. Fearon, Tulsa, Okla,

assignors. to Well Surveys, Incorporated, a corporation of DelawareOriginal application-July '30, 1949, Ser. No. 107,806,

now Patent No. 2,712,081, dated June 28, 1955. Divided and thisapplication June 24, 1955, Ser. No. 517,764 j Claims. (Cl. 250.84.5)

This application is a division of our copending application Serial No.107;8Q6,'filed J-uly 30, 1949, for Method for Neutron Well Logging;

This copending application, now US. Patent No. 2,712,081, issued June28, 1955, relatw generally to a method and apparatus for identifyingsubstances existing in difiicultly accessible locations, for example,adjacent to a deep narrow drill hole, and more particularly to a methodand apparatus for identifying and distinguishing these substances fromeach other by nuclear reactions in the substances.

The original application related more particularly to methods andapparatus for identifying substances in subsurface geological formationsby the specific energy losses of neutrons striking the formations. Inorder to derive meaning from such measurements as were contemplated, itwas necessary to provide a calibrating or adjusting system.

The primary object of this invention is therefore to provide a method ormeans for determining specific energy losses in samples of substancesexposed to neutrons of a determined energy for the purpose of adjustingenergy selective neutron detector systems used in well logging. Afurther object of this invention is to provide a method and apparatusfurnishing neutrons of a determined energy, which energy may be selectedover a Wide range.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when considered withthe drawing, in which:

The sole figure illustrates diagrammatically means for obtaininginformation about the absorption in a sample of neutrons having aparticular energy.

In the sole figure a system is illustrated for determining the existenceand location of neutron resonances for interaction between nucleicontained in a sample and a stream of bombarding fast neutrons. Theseneutron resonances will correspond with specific energy losses which canbe used in considering the adjustment of a detector of fast neutronradiation such as was set forth in connection with FIGURE 8 of ourcopending application now US. Patent No. 2,712,081. In this figure thereis shown a source of monoenergetic fast neutrons 67 almost surrounded bya mass of paraflin 68. This source of monoenergetic fast neutrons may besuch as described in said copending application. Through this mass ofparafiin there is provided a narrow opening 69 from which emerges anarrow beam of neutrons 70 which impinges upon a mass of hydrogenousmaterial 71, such as paraflin. The hydrogen of material 71 serves toscatter elastically neutrons derived from the beam 70. The diameter ofthe hydrogenous mass 71, which may be spherical in shape, issufliciently small that neutrons of the beam 70 will generally bescattered by having only one collision with a hydrogen nucleus in themass 71 rather than a plurality of collisions which would occur if thediameter of mass 71 is excessive. The exact dimensions of the materialdepend upon the energy of the incident neutrons and how pure it isdesirable or necessary to keep the neutron energies at the variousangles. Obviously, a greater thickness makes it more certain that theincident neutrons of beam 70 strike at least one hydrogen nucleus andnot be wasted by passing directly through the material 71. Theinteraction between neutrons and hydrogen nuclei is amatter ofprobability, and there are no particular dimensions of material 71 thatprovide results most desirable in all cases. 1 From the physicalconstants involved, it is demonstrable that a thickness of the order ofone milli meter is satisfactory. A detector 72 is located for andadapted to rotation about mass 71 as a center. A sample 73 is situatedat a constant distance from the detector 72 and maintained always on adirect. line joining the center of the detector and the center of thescatteringmass 71. The 'angle of rotation determines exactly the loss ofenergy which a neutron 'from the beam 70 will suffer in being deflectedby the object 71 in the direction of the detector 72. Since all theneutrons of the beam 70 have equal energies the neutrons passing throughthe sample and in through the detector will also have, at theirincidence upon the sample, equal but smaller energies, smaller than theenergies of the neutrons in the beam 70. The energies of the neutronsincident upon the sample will, accordingly, vary from the energy of thesource for 0=180 to practically nothing for 0:0, and there will be foreach value of 0 a corresponding value of the energy within the aboverange. One may accordingly restate 0 as a scale of energy and plot thetransmission of neutrons through the sample as a function of energy byvarying 0. If it is assumed that the mass of a neutron is equal to themass of a proton, from the principles of conservation of momentum andenergy it is clear that the relationship between energy and scatteringangle is E=E cos (1r0) where E is the energy of neutrons scattered at anangle 0,

E is the energy of incident neutrons, and

0 is the angle between the direction of the scattered neutrons and thedirection whence came the incident neutrons.

Minima in the said function of energy will correspond with a specificreaction in the sample suffered by the neutrons at that energy.

It is to be understood that this invention is not to be limited to thespecific modifications described, but is to be limited only by thefollowing claims.

We claim:

1. A source of fast neutrons of continuously adjustable predeterminedenergy comprising a source of substantially monoenergetic fast neutrons,a hydrogenous shield for passing only a narrow beam of saidmonoenergetic fast neutrons, and a thin hydrogenous target disposed insaid beam whereby said monoenergetic fast neutrons are scattered in alldirections with predetermined energy for each angle of scattering.

2. A source of fast neutrons of continuously adjustable predeterminedenergy comprising a source of substantially monoenergetic fast neutrons,a hydrogenous shield for passing only a narrow beam of saidmonoenergetic fast neutrons, and a hydrogenous target disposed in saidbeam so thin that neutrons are scattered thereby substantially only oncewhereby said monoenergetic fast neutrons are scattered in all directionswith predetermined energy for each angle of scattering.

3. A source of fast neutrons of continuously adjustable predeterminedenergy comprising means for producing a beam of substantiallymonoenergetic fast neutrons, a thin hydrogenous target disposed in saidbeam whereby said monoenergetic fast neutrons are scattered in alldirections with predetermined energy for each angle of scattering, andmeans for selectively utilizing neutrons scattered from said target at aparticular angle whereby neutrons of a particular energy are selected.

4. A source of fast neutrons of continuously adjustable predeterminedenergy comprising a source of substantially monoenergetic fast neutrons,.a lgydroggngus, shield for passing only a narrow beam of said monoenergetic fast neutrons, a thin hydrogenous' target disposed in said beamwhereby said monoenergetic fast neutrons are scattered in all directionswith predetermined energy for each angle of scattering, and means forselectivelyutilizing neutrons scattered from said tar-get-ata selectedangle whereby neutrons of a particular energy are selected.

5. Apparatus for providing neutrons: of adjustable energy comprisingmeans producing a bean; jof substantially monoenergetic fast neutrons,anihydrogenous target disposed in said beam and ofsuel; dimensions thatneutrons in said beam are scattered thereby substantially o y o ce, a dm ans for sal s ,ivelyv ut li in neut qn scattered at at least oneparticular angle whereby neutrons of a particular energy are selected.

References Cited in the fileiof this patent UNITED STATES PATENTS OTHERREFERENCES Zinn.: Physical Review, Vol.71, No. 11 June 1, 1947, pp.752-756.

Chagnon et al.: Review of Scientific Instruments, vol. 24, No. 8, August1953, pp. 656 110.660.

